The present invention relates to apparatus of both the selector switch type and the pre-selector type wherein a tap changer comprises two connection branches each connected between a common point of connection and its separate movable contact, with at least one branch containing a resistor in series with a contact device. The other branch may or may not include a vacuum element. The two movable contacts are successively displaceable between a number of fixed contacts intended for connection to the tappings of a regulating transformer.
It has been proposed previously to use vacuum elements in tap changers for the purpose of, among other things, achieving a relatively high breaking capacity and avoiding the release of gas and contamination of the oil surrounding the contact devices. However, the vacuum elements now available do not withstand any mentionable overcurrents. In a previously proposed tap changer of the abovementioned type, a bridging contact device was inserted in the resistor branch (British Patent Specification No. 1,197,379), thus making possible a division of the current between the two connection branches. A condition for this, however, is that the movable contacts of the connection branches are in good electrical connection with the fixed contacts connected to the tappings of the transformer winding. For example, wear, oxidation or contamination of the contact surfaces may change the contact conditions with time, so that the current will flow substantially only through one of the connection branches. Therefore, in order not to jeopardize the security of operation, both the vacuum element and the bridging contact device must be dimensioned for the full rated and short-circuit current of the transformer, and the stated advantage of the known connection are not fully obtained.
Other previously proposed embodiments with a vacuum element (for example, according to British Patent Specification 1,164,782) also involve certain drawbacks, either because of the great number of required components (for example, two resistors and six contact devices per phase) thereby necessitating a complex operating device, or because the whole transformer current continuously passes through the vacuum element. In the latter instance, elements with great current conducting capacity are required so that the construction thereof is unrealistic from a technical-economic viewpoint.